Gas company Inpex has been fined $12,600 for evaporating water containing PFAS over Darwin Harbour.

Inpex was found to have released the potentially toxic chemical from its new $37 billion LNG processing plant site on the harbour.

The Federal Environment and Energy Department has warned that the release of the PFAS in September last year put wildlife, including dolphins, and dugongs at risk.

Inpex’s environmental management plan requires it to store water containing PFAS (per-fluoroalkyl and poly-fluoroalkyl substances) from firefighting exercises on site, and transport it interstate for treatment at a licensed facility.

However, the Department found that “in September 2018 Inpex boiled wastewater from firefighting tests that contained PFAS”.

Those actions are “"likely to have released PFAS into the air and therefore may have placed the immediate environment, including Darwin Harbour, at risk of heightened PFAS levels’, the department said.

“Darwin Harbour is habitat for EPBC Act listed threatened and migratory species including dugongs, multiple cetacean species, two sawfish species, saltwater crocodiles and multiple sea turtle species,” it said.

It fined Inpex $12,600.

Inpex said its firefighting tests had generated a large amount of wastewater, containing a small amount of PFAS.

The company claimed its process of “enhanced evaporation” was used to minimise the load of wastewater it would be required to truck interstate.

“Enhanced evaporation was used to reduce the amount of wastewater needing to be transported,” it said.

The company paid the fine while continuing to claim complied with its environment management plan.

“At no stage in this process did wastewater leak into the external environment,” it said.

“Inpex considers this process was done in accordance with all approvals but nevertheless made the requested payment when a notice was subsequently received.”

The Federal Environment Department's investigation was only revealed by a Freedom of Information by the Australian Conservation Foundation.

“It's good that the Federal Environment Department is cracking down on toxic pollution spills, which are a threat to precious wildlife, but a $12,600 fine clearly does not send an adequate signal to business,” the foundation's nature program manager Jess Abrahams said.

The Department said the fine was “proportional enforcement relative to the offence and (Inpex's) compliance history”.

The official documents also noted that news of the PFAS release would be controversial in a city already afflicted with PFAS contamination.

Reports say an inquiry by the Northern Territory Environment Protection Authority into the PFAS release by Inpex is ongoing.